This disclosure relates generally to a cholesteric writing/drawing tablet, for example, utilizing a pressure sensitive display. In general, Bistable Liquid Crystal Displays, and in particular, Cholesteric Liquid Crystal Displays (ChLCDs), have proven to have great potential to create low cost pressure sensitive displays that are efficient power consumers and that can be utilized in a number of unique devices.
Recently, the pressure sensitive cholesteric liquid crystal writing tablet, BOOGIE BOARD® of IMPROV® Electronics has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. This tablet offers a considerable improvement over previous tablet technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrodes in the tablet. In a cholesteric liquid crystal writing tablet, the liquid crystal, dispersed in the polymer network, is sandwiched between two substrates that are spaced to a particular gap. The upper substrate is flexible and the bottom substrate is painted with an opaque light absorbing dark background. Within the gap is a bistable cholesteric liquid crystal which can exhibit two textures, a substantially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The writing tablet is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the substantially transparent texture. When one presses on the top substrate with a pointed stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from substantially transparent to a brilliant reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to transparent conducting electrodes on the inner surface of the substrates that drive the cholesteric liquid crystal from its color reflective state back to its substantially transparent state. A plurality of these cholesteric writing tablets discussed here can be modified for use in the cholesteric writing board of this disclosure.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448. Polymer dispersions can be used to control the pressure sensitivity and resolution of the image as described in U.S. Pat. No. 8,228,301, which is suitable for use in the cholesteric writing board of this disclosure and is incorporated herein by reference in its entirety. Other modes of operation and a writing tablet that can produce multiple color images are described in this U.S. Pat. No. 8,228,301, and a means for select erase is disclosed in U.S. Pat. No. 8,139,039, incorporated herein by reference in its entirety, all of which are suitable for use in the cholesteric writing tablets of the cholesteric writing board of this disclosure. One mode of operation provides a negative image from that described above. In that mode the tablet is initialized by electrically driving the tablet display to the color reflective texture with a voltage pulse or pulses. Then with a continuous voltage applied to the electrodes of an appropriate value, one can write images by driving the cholesteric material to the substantially transparent texture with the pressure of a pointed stylus. This mode of operation with a color reflective background is termed Mode A whereas the other mode with a transparent background is termed Mode B.
The commercial BOOGIE BOARD® writing tablet, operated in Mode B, has the color black for the fixed opaque light absorbing background. The dark black background offers high contrast for the color reflective image written on the tablet. As disclosed in U.S. Pat. No. 5,493,430, incorporated herein by reference in its entirety, other opaque colors may also be used for the fixed background of a cholesteric liquid crystal display. The color of the background additively mixes with the reflective color to present a different color than that of the cholesteric liquid crystal. There may be multiple colors on the background and those colors may be patterned. As an example, the pattern could be lines offering a lined tablet for convenience in writing text similar to a ruled paper tablet.
A problem with the prior art is that the size of a commercial writing tablet or eWriter, has been constrained by manufacturing to no larger than about 11 inches on the diagonal. This is too small for use as an electronic board or eBoard in replacement for a blackboard or white board typically used in a classroom or conference room. An approach to this problem is to tile several of the small size eWriters together similar to what has been done with digital electronic LCD displays for numerous applications, including; large viewing area command stations, billboards, and 3D graphics devices for virtual environments, among others.
However, the inventors have learned about the following problems when attempting to tile cholesteric writing tablets together into a larger display of this disclosure. One cannot simply tile a pressure driven tablet display in the same way as other prior art electronic LCDs since the electronic LCDs are made of glass rather than plastic substrates; they are pixelated for electronic addressing whereas the tablet is continuous and driven with the pressure of a stylus. The edges of the tiles where they intersect with an adjacent tile can leave bumps and cause drag on the stylus. Furthermore, in a pressure driven writing tablet, the edge of the tiled eWriter display can affect the appearance of the traced line if not suitably tiled. Another problem that arises is the width of the pressure written line. An eBoard is viewed at a distance and the inventors herein have found it is desirable for the width of the written line to be broader than that of the BOOGIE BOARD® tablet. Still another issue found by the inventors herein is line acuity when writing on a vertically mounted surface (similar to a blackboard) as opposed to a horizontal surface (similar to a tablet). The tiling of eWriter displays herein is designed not only to hide the seams but to allow for writing with broad tipped pens and to allow for gray scale images when desired.